This invention relates to a removably coated optical waveguide and in particular to a removably coated optical fibre for laser transmission which may be incorporated into an optical device or telecommunication system.
Since their inception in the mid-1960""s, the growth of the use of optical fibres, generally manufactured from silica or other glasses, in optical devices and telecommunication systems has been substantial. This growth has been due largely to the remarkable data carrying capacity displayed by fibres.
Generally speaking the optical fibre comprises an optical fibre core, of a certain refractive index, which may be clad by an optical cladding of slightly lower refractive index. Alternatively, a graded index fibre, which comprises layers of optical material with decreasing refractive indices, may be employed.
However, optical fibres do have a significant draw back, in that they are relatively brittle and fragile. Therefore, optical fibres are commonly provided with a protective coating of a plastics material, commonly a UV curable material such as an acrylate or a UV curable polymide. Desolite 950-106, 950-108, 950-131 and 3471-3-14 are routinely used for coating optical fibres for general purpose uses. Thermally cured silicon coatings, such as Sylgard 180 and 184 are also available. However, these materials provide a relatively soft and tacky coating which is used in either high temperatures or chemical environments in which acrylate coatings are not suitable.
For practical use in telecommunication systems such coated fibres may be combined into multi-fibre cables formed, for example, from a polyurethane outer jacket. If additional strengthening is required the jacket may be provided with strengthening members formed, for example, from Kevlar. However, as this application relates to the coating of individual fibres, such multi-fibre cables will not be discussed further herein.
In use optical fibres tend to be end coupled and end pumped, at least in part because of the requirement to coat the fibres. That is, any useful radiation within the optical fibre is injected through the end face of the fibre, travels the full length of the fibre through internal reflection, in the normal manner, and is transmitted from the remote end of the fibre.
However, in some situations there are significant advantages to be gained from direct access to uncoated portion of the optical fibre. By far the most important of these is the use of a light source to produce permanent light sensitive changes within the optical fibres. In other words advantages are gained from the use of a light source to produce a periodic refractive index modulation within the fibre in the form of a grating. The most common light source used is a UV laser such as an Eximer or Argon ion laser. Such gratings can be used in a host of devices from sensors and band pass filters to fibre laser mirrors.
Applications which require the removal of the coating over lengths exceeding 50 mm include, for example: fibre grating dispersion equalisers and long period gratings, which may be written into the optical fibre (either the core or the cladding), for applications which include gain flattening, polarisation rocking and or loss filters; wave length selective polarisers; and cladding mode pumped amplifiers. For example, a dispersion equaliser requires up to eight meters in length of coating to be removed from the fibre, which leaves a substantial length of optical fibre which is open to contamination or physical damage.
In this device a long chirped grating is written into a stripped fibre spanning the 32 nm gain window of an erbium amplifier. Such a device, when connected to the first output port of a three port circulator provides equalisation of dispersed signals transmitted down the fibre. The chirped grating simply removes the wavelength dependent time delay spreading arising from the dispersion in the transmission fibre.
Long period grating applications operate on the basis that a refractive index variation along the length of a fibre has a period that matches the difference in propagation constants for the lowest order mode in the fibre core and some higher order mode in the cladding. This causes power to be transferred from one mode to the other, which can be used to provide wavelength selective loss and polarisation rocking in two moded fibres, ie alternate coupling between the two polarisation modes that exist in so called single mode fibres. In cladding mode pumped amplifiers by writing the index changes in the cladding power can be transferred from the cladding to the core more efficiently than would otherwise be possible.
The invention would be useful for fused couplers where two or more optical fibres are melted together to enable light to be transferred between the respective cores. This process requires the absence of any polymer coating or residue on the fibres.
A further area in which the invention may be useful is in the area of non-intrusive taps wherein a loss mechanism such as micro-bending can be used to eject light from the length of the fibre. If the coating is removed this light can more easily and efficiently be accessed. Therefore, there is a conflict between the need for access to the fibre and the need for protective coatings.
At present, access to lengths of optical fibre is obtained either by dissolving the fibre coating in a suitable organic solvent or the removal of a length of coating using a blade, to scrape away the coating. The problem with the latter approach is that it can damage the fibre or its cladding. Also, the process of dipping the fibre in solvent and washing an drying the fibre thereafter can also result in fibre damage.
Neither of these processes are of real practical value when substantial lengths of fibre greater than a few centimeters need to be stripped. Particularly, the use of a blade to scrape away the protective coating causes significant stress on the fibre which can cause an unacceptable degree of damage.
It is an object of the present invention to provide a coating for a fibre through which access to the fibre can be obtained, easily and conveniently, with minimal probability of damaging the fibre during the stripping process.
According to a first aspect of the present invention there is provided a removably coated optical fibre comprising:
(i) an optical fibre; and
(ii) a removable coating formed from an elongate tape having two edges, the tape being affixed adjacent to said edges along a substantial length of the tape to form a releasable seam, access to the fibre being obtainable by peeling apart said seam.
According to a second aspect of the present invention there is provided a removably coated optical fibre comprising:
(i) an optical fibre; and
(ii) a removable coating formed from first and second elongate tapes which are affixed on either side of said optical fibrealong a substantial length of said tapes to form a releasable seam, access to the optical fibre being obtainable by peeling apart said tapes.
The coating may thus be readily and easily removed from the optical fibre by peeling apart along the releasable seam. The seam is manufactured in such a way as to provide a relative weak point in the coating, as compared with the strength of the tapes or other material used to make the coating once access to the fibre has been initiated, relatively large sections of the fibre may be exposed, without damage to the fibre, simply by peeling, pulling or tearing the seam.
Preferably, the coated optical fibre comprises a lubricant provided between said coating and said fibre.
Most preferably, the lubricant is a fluid, for example, ethylene glycol.
Alternatively, the lubricant is a solid, for example, a layer of polytetraflurethene (PTFE) applied to the inner.
Preferably, the tape is formed from low density polyethelene.
Preferably, the tape is sealed adjacent said edges using an adhesive such as a latex based pressure sensitive adhesive.
Preferably, the first elongate tape is planar, said fibre resting thereon, and said second elongate tape is moulded so as to provide an elongate recess for receipt of said optical fibre.
Most preferably, the elongate recess has a substantially xe2x80x9cUxe2x80x9d-shaped cross-section.
Alternatively, both said first and second tapes comprise an elongate recess arranged such that when the tapes are co-joined a recess is formed for receipt of said optical fibre. In this embodiment it is most preferably that the recesses are both substantially semi-circular in cross section.
Alternatively, both the said first and second tapes are planar.
In this embodiment it is preferable that an additional tape is placed on either side of the optical fibre between the first and second planar tapes.
According to a third aspect of the present invention there is provided a method of manufacture of a removably coated optical fibre including an optical fibre and a removable coating, the method comprising the steps of:
(a) passing an elongate coating tape around a portion of a roller;
(b) passing an optical fibre around said roller so as to rest on said tape; and
(c) passing said tape and optical fibre combination through a sealing means to releasably seal the tape adjacent to said edges, so as to enclose said optical fibre within said removable coating, thereby forming said removably coated optical fibre.
According to a fourth aspect of the present invention there is provided a method of manufacturing a removably coated optical fibre including an optical fibre and a removable coating; the method comprising the steps of:
(a) passing a first coating tape around a portion of a roller;
(b) passing said optical fibre around said roller so as to rest on said first tape;
(c) passing a second coating tape around said portion of the roller, on the opposite side of said optical fibre so as to form said removably coated optical fibre.
Preferably, the fibre is passed through a lubricant applicator prior to combination with said tape, at said roller.
Preferably, the lubricant applicator is a bath of ethylene glycol.
Preferably, the fibre is obtained directly from a fibre drawing process.
Alternatively, the fibre is a partially coated fibre obtained directly from a partial coating process.
Preferably, when two tapes are utilised, the first and second coating tapes are stored on separate first and second tape drums, the roller being located on route between said first and second drums and the collection drum.
Most preferably, the first coating tape is moulded to provide a recess to receive said optical fibre, said recess, for example, having a substantially unshaped cross section.
In one embodiment both said first and second tapes may be moulded prior to combination so that the combined tapes provide a recess for receipt of said optical fibre.
In this case, both said first and second tapes have an elongate recess with, for example, a substantially semi-circular cross section.
In a further embodiment, the first and second tapes may be planar.
In this case, an additional tape may be placed on either side of the optical fibre between the planar tapes.
Preferably, said first coating tape is provided on a base tape.
Preferably, said first and second coating tapes are formed from low density polyethelene and said base tape is formed from Kevlar.
Preferably, the rate of accumulation of the removably coated optical fibre on a collector drum is controlled by an accumulator arm placed between the roller and the collector drum.
Preferably, the edges of the tapes are releasably sealed by a sealing means located between the roller and the collector drum.
According to a fifth aspect of the present invention there is provided an apparatus for the manufacture of a removably coated optical fibre comprising an optical fibre; and a removable coating, the apparatus including:
(a) a storage drum for the storage of an elongate coating tape;
(b) means of accessing an optical fibre either directly from the fibre pulling process or alternatively from a fibre partial coating process;
(c) a collection drum to collect and store the removably coated optical fibre;
(d) a roller positioned on route between said storage drum and said collection drum arranged such that said tape and said fibre core are passed around a portion of the roller; and
(e) means arranged to enclose the tape around the fibre.
According to a sixth aspect of the present invention there is provided an apparatus for the manufacture of a removably coated optical fibre comprising an optical fibre and a removable coating, the apparatus including:
(a) a first tape drum for the storage of an elongate coating tape;
(b) means of accessing an optical fibre either directly from the fibre pulling process or alternatively from a fibre partial coating process;
(c) a collection drum to collect and store the removably coated optical fibre;
(d) a roller positioned between said drums and arranged such that said tape and said optical fibre are passed around a portion of the roller; and
(e) a second tape roller arranged to provide a second tape on the other side of the fibre from the first tape.
Preferably, a further roller carrying a base tape is arranged to position the base tape beneath the first coating tape.
Preferably, a bath of lubricant is positioned between the means for accessing the fibre and the roller wherein the lubricant is ethylene glycol.
Preferably, the tape and optical fibre combination pass through a sealing means which releasably seals the edges of the tape.
Preferably, an accumulator arm is placed between the roller and the collector drum in order to control the rate of accumulation of removably coated optical fibre on the collector drum.
Preferably, a moulding stage is provided between the first tape drum and the roller in order to mould the first tape to provide an elongate recess therein with, for example, a substantially xe2x80x9cUxe2x80x9d-shaped cross-section to receive the optical fibre.
In addition, a moulding stage may be provided between each tape drum and the roller in order to mould the tapes to provide each tape with an elongate recess therein with, for example, substantially semi-circular cross-sections which, when the tapes are combined, provides a recess to receive the optical fibre.
Alternatively, the first and second tapes may be planar wherein a means is provided for arranging an additional tape on either said of the optical fibre.
According to seventh aspect of the present invention there is provided an optical fibre transmission system comprising an assembly of at least a first and second optical fibre and an associated optical fibre coupler which couples the first and second optical fibres together, said first optical fibre comprising:
(i) an optical fibre having a grating written therein; and
(ii) a removable coating formed from a first and second elongate tapes which are releasably affixed on either side of said optical fibre, access to the grating in the optical fibre being obtainable by peeling apart said tapes.
Preferably, the second optical fibre is a long-haul optical fibre having kilometer order lengths and the first optical fibre is of meter order lengths or less.
Preferably, the grating is a UV laser written grating.
According to an eighth aspect of the present invention there is provided an optical fibre telecommunications system comprising a laser pulse signal input device, an optical transmission line which comprises a removably coated optical fibre and which is optically coupled to the laser pulse signal input device and a receiver means for receiving laser pulse signals output from the optical transmission line to enable transduction of the signal carried by the laser pulses, said removably coated optical fibre having a grating written therein, the removably coated optical fibre comprising:
(i) an optical fibre; and
(ii) a removable coating formed from first and second elongate tapes which are affixed on either side of said optical fibre core, along a substantial length of said tapes to form a releasable seam, access to the fibre being obtainable by peeling apart said seam.
According to a ninth aspect of the present invention there is provided an optical device comprising a removably coated optical fibre in a grating is written, the grating being selected from the group consisting of:
(a) a grating oriented normal to the direction of travel of light to be transmitted along the fibre grating acting as a dispersion equaliser; and
a grating oriented non-normally and non-aligned to the direction of travel of the light to be transmitted along the fibre, the grating acting as a non-intrusive radiation mode tap,
the removably coated optical fibre comprising
(i) an optical fibre; and
(ii) a removable coating formed from first and second elongate tapes which are affixed on either side of said optical fibre core, along a substantial length of said tapes to form a releasable seam, access to the fibre being obtainable by peeling apart said seam.